Rechargeable lithium-ion batteries have received extensive attention in the last two decades and have been used in portable electronic devices such as laptop computers, cellular phones and personal digital assistants. However, application to electric vehicles and grid energy storage is limited by performance and cost. The main components of battery costs are materials, labor and overhead with the cost of materials and associated processing making up over 80% of total costs of high power batteries. Thus, the key to reducing costs of lithium-ion batteries lies in achieving low cost materials and developing low cost material processing, which is especially true for the cathode. Cathode materials and processing represent the majority of the total cost of high power batteries.
LiFePO4 is a promising cathode material for the next generation of scalable lithium-ion batteries, which is ascribed to low price, good cycle life, safety and low environmental impact (i.e. no toxic elements in the compound). For conventional lithium-ion batteries, the manufacturing process of LiFePO4 cathodes involves a slurry processing in which LiFePO4 is mixed with other additives in a solvent. Polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (NMP) are the typical binder and solvent, respectively. If the composite cathodes could be processed through an aqueous system, in which the expensive NMP is replaced with deionized water, the cost would be significantly reduced and the process for recovery and treatment of NMP would be eliminated. Additionally, replacing PVDF with xanthan gum or carboxymethyl cellulose would reduce fluorine content in the electrodes, and the formation of LiF could be suppressed. The overall process would become substantially more environmentally benign; consequently, there is growing interest in fabricating composite cathodes through aqueous processing. However, replacing NMP with water creates problems with dispersion stability. Particles in water based dispersions can agglomerate due to hydrogen bonding and strong electrostatic forces. These driving forces are even more problematic for LiFePO4 since the material is optimized for improved electrochemical performance by making nanoparticles with a resulting larger surface area.
Agglomeration is caused by the interactions between colloidal particles. These interactions include attractive and repulsive potentials, which are generated from van der Waals and Coulomb forces, respectively. Usually, the attractive potential is dominant at greater distances between particles. The stability of the particles depends on the net potential generated between the van der Waals and Coulomb forces. Therefore, to minimize agglomeration, the key is to increase the repulsive potential (i.e. increase the Coulomb force) between particles. The repulsive potential depends on the particle surface charge, and it is measured indirectly. The measurement is known as the zeta potential and it is dependent on the surface chemistry of colloidal particles.